WO2006114501A1

WO2006114501A1 - Use of polysaccharides in order to eliminate heavy metals in the form of anions from water

Info

Publication number: WO2006114501A1
Application number: PCT/FR2006/000889
Authority: WO
Inventors: Caroline Mabille; Vincent Monin; Yves Mottot; Jean-François SASSI
Original assignee: Rhodia Chimie
Priority date: 2005-04-28
Filing date: 2006-04-21
Publication date: 2006-11-02
Also published as: FR2885125A1; US20090272693A1; EP1879834A1; CN101166694A; FR2885125B1; CA2607452A1; KR20070116274A; KR20110031390A

Abstract

The invention relates to a simple method of purifying water loaded with heavy metals in the form of anions. The inventive method is based on the use of a composition containing at least one polysaccharide, such as starches or vegetable gums.

Description

USE OF POLYSACCHARIDES FOR REMOVAL OF HEAVY METALS CONTAINED IN THE FORM OF ANIONS IN WATERS.

The invention relates to the field of water treatment, in particular to the elimination of metals present in the form of anions in water and more particularly to the elimination of arsenic contained in natural, industrial and waste water. .

Some metals found in water can cause many health problems because of their toxicity. The metals present in natural waters are mainly of natural origin. For example, arsenic comes from the dissolution of arsenic As (III) or As (V) present in the rocks surrounding the groundwater. In some regions of the world, the concentration of arsenic present in natural waters can reach values of a few hundred μg / l.

The elimination of toxic metals such as arsenic, antimony, tin, vanadium, germanium, molybdenum and tungsten from water is therefore an essential objective to ensure the quality of drinking water produced from natural waters. In Europe, the European Directive 98/83 EC of 3 November 1998 thus imposes for the drinking water an arsenic rate lower than 10 μg / l and antimony lower than 5 μg / l, this limitation being also recommended by the 'World Health Organization.

Until now, to eliminate arsenic, it was known to use alumina alone. It has also been described in patent CA 1067627 the possibility of using an oxide and / or hydroxide of iron previously deposited on a support including alumina. However, one of the disadvantages of this system is the need to prepare beforehand an iron hydroxide product on alumina. In addition, if the amount of iron hydroxide deposited on the alumina is not important enough and if there is then a defect of presence of iron hydroxide in contact with the alumina, it is not possible to add iron hydroxide during the process. The need existed to find a way to remove metals such as arsenic which does not have the particular disadvantages mentioned above.

One of the aims of the present invention is therefore to find a means of eliminating metals such as arsenic which may in particular make it possible to obtain a retention greater than the means known hitherto.

Another object of the present invention is to provide a means of removing metals such as arsenic from water which is inexpensive in terms of investment and exploitation.

The Applicant has discovered a means of purifying the water by a simple method meeting the objectives described above, which consists in bringing the water to be purified into contact with a polysaccharide that is particularly well suited.

The first object of the invention is therefore the use of a composition comprising at least one polysaccharide for purifying a water loaded with metals.

According to the use of the invention, the metals to be removed, generally present in the form of anions in water, are chosen from the group consisting of arsenic, antimony, tin, vanadium and germanium. , molybdenum and tungsten. More preferably the use of the invention is applied to the removal of arsenic.

The form in which arsenic is in aqueous solution is strongly pH dependent. For I ¹ As (V), it is in neutral form at pH <3, then anionic beyond. L ¹ As (III) is - meanwhile - in cationic form at pH <2, neutral between 2 <pH <9 and anionic beyond. No particular limitation is imposed on the polysaccharides to be used according to the invention. All those described by Progress in Polymer Science 30 (2005) 38-70 may be used as an indication.

According to one particular form of the invention, the polysaccharide is chosen from the group comprising cellulose, starches and vegetable gums.

The cellulose may be of any origin, for example of plant, bacterial, animal, fungal or amoebic origin, preferably plant, bacterial or animal origin. Examples of vegetable sources of cellulose include wood, cotton, flax, ramie, certain algae, jute, waste from the food industry or the like. Examples of animal sources of cellulose include animals from the tunicate family.

The starch may be selected from wheat starch, potato starch, corn starch, sweet potato starch, tapioca starch, cassava starch, starch sago, rice starch, glutinous corn starch, waxy maize starch, high amylose corn starch or mixtures thereof. The starch can be used as it is or after having undergone pretreatment pre-gelatinization such as cooking with hot water or steam. Preferably the starch is corn, wheat or potato.

No particular limit is imposed on the purity of the starch. In this sense, natural flour rich in starch can also be used, such as cereal flour such as wheat, corn, or potato starch.

The term "starch" used hereinafter refers to purified starches as well as natural flours.

No particular limitation is imposed on the vegetable gum used in the invention, and examples of vegetable gums that can be used include glucomannans such as Konjac, xyloglucans such as Tamarind gum, galactomannans such as guar, carob, tara, fenugreek or "mesquite" gum, or gum arabic, or mixtures thereof. Preferably, galactomannans and in particular guars are preferred.

No particular limit is imposed on the purity of the vegetable gum. In this sense, natural flour rich in vegetable gum can also be used, such as for example native guar powder or native carob powder without any refining or their mixtures.

The term "vegetable gum" used subsequently refers to both purified vegetable gums and natural flours.

According to one embodiment of the invention the polysaccharide is optionally modified to improve its affinity for the metals to be eliminated, and thus improve its ability to capture these metals on the one hand and make it insoluble on the other hand which allows the separate more easily from the liquid solution to be treated. These modifications intended to improve the affinity of the polysaccharide and to make it insoluble can be carried out separately and in the order that is desired. It may also be possible to make these changes simultaneously.

Among the modifications to be carried out, mention may be made of the introduction of cationic or cationizable groups. By cationizable groups is meant groups which can be made cationic depending on the pH of the medium. (Preferred pH: for example pH> 9 for tertiary amine functions).

Among the cationic or cationizable groups, there may be mentioned groups comprising quaternary ammoniums or primary, secondary or tertiary amines, pyridiniums, guanidiniums, phosphoniums or sulfoniums.

The cationic modified polysaccharides used in the invention can be obtained by customarily reacting the polysaccharide raw materials mentioned above. The introduction of cationic or cationizable groups into the polysaccharide may be carried out by a nucleophilic substitution reaction.

In the case where it is desired to introduce an ammonium group, the adapted reagent used may be:

- 3-chloro-2-hydroxypropyltrimethylammonium chloride, sold in particular under the name QUAB 188 by the company Degussa;

an epoxide carrying a quaternary ammonium such as 2,3-epoxypropyltrimethylammonium chloride, sold in particular under the name QUAB 151 by the company Degussa, or analogous compounds;

diethylaminoethyl chloride; or Michael's acceptors such as, for example, acrylates or methacrylates carrying quaternary ammonium or tertiary amines.

The introduction of cationic or cationizable groups into the polysaccharide can be carried out by esterification with amino acids such as, for example, glycine, lysine, arginine, 6-aminocaproic acid or with quaternized amino acid derivatives. such as, for example, betaine hydrochloride.

The introduction of cationic or cationizable groups into the polysaccharide can also be carried out by a radical polymerization comprising the grafting of monomers comprising at least one cationic or cationizable group on the polysaccharide.

Radical priming can be carried out using cerium as described in the European Polymer Journal publication vol. 12, p535-541, 1976. Radical priming can also be carried out by ionizing radiation and in particular by bombardment. electron beam.

The monomers comprising at least one cationic or cationizable group used to carry out this radical polymerization may be by for example monomers comprising at least one ethylenic unsaturation and at least one quaternary or quaternizable nitrogen atom by adjusting the pH.

Among these monomers comprising at least one ethylenic unsaturation and at least one quaternary nitrogen atom or quaternizable by adjusting the pH can be mentioned the compounds of formulas (I), (II), (III), (IV) or (V) following:

the compound of general formula (I)

(D in which:

- A ^nΘ represents a CI ion ^Θ, Br ^Θ, ^Θ I, SO ₄ ^2Θ, ^2Θ CO _3, CH ₃ -OSO ₃ ^0, ^Θ 0H or CH ₃ -CH ₂ -OSO ₃ ^0, - R ¹ to R ⁵ are identical or different represent, independently of one another, an alkyl group having 1 to 20 carbon atoms, a benzyl radical or an H atom, and

n is 1 or 2, or

the compound of general formula (II)

(he) in which :

X represents a group -NH or an oxygen atom O,

R ⁴ represents a hydrogen atom or an alkyl group having from 1 to 20 carbon atoms,

R ⁵ represents an alkene group having from 1 to 20 carbon atoms,

R ¹ , R ² and R ^{3, which are} identical or different, represent, independently of one another, an alkyl group having from 1 to 20 carbon atoms,

- B ^NΘ represents a CI ion ^Θ , Br ^Θ , I ^Θ , SO ₄ ^2Θ , CO ₃ ^2Θ , CH ₃ -OSO ₃ ⁰ , OH ^Θ or CH ₃ - CH ₂ -OSO ₃ ⁰ , and

n is 1 or 2, or

the compound of general formula (III)

(III)

in which :

R ¹ to R ^{6, which are} identical or different, represent, independently of one another, a hydrogen atom or an alkyl group having from 1 to 20 carbon atoms, but with one of the groups R ¹ to R ⁶ representing a group -CH = CH ₂ ,

- C ^NΘ represents a CI ^® , Br ^Θ , I ^Θ , SO ₄ ^2Θ , CO ₃ ²⁰ , CH ₃ -OSO ₃ ⁰ , 0H ^Θ or CH ₃ - CH ₂ -OSO ₃ ⁰ , and - n is 1 or 2 , or

the compound of general formula (IV)

(IV)

in which: - D represents a CI ion ^nΘ ^® ^Θ Br, I ^0, SO ₄ ²⁰ ^2Θ CO _3, CH ₃ -OSO ₃ ^0, ⁰ OH or CH ₃ - CH ₂ -OSO ₃ ^0, and - n is 1 or 2.

Preferably, the monomers comprising at least one ethylenic unsaturation and at least one quaternary or quaternizable nitrogen atom are chosen from: 2-dimethylaminoethyl acrylate (ADAM), quaternized 2-dimethylaminoethyl acrylate (ADAM-Quat), 2-dimethylaminoethyl methacrylate (MADAM), quaternized 2-dimethylaminoethyl methacrylate (MADAM-Quat), quaternized 2-diethylaminoethyl methacrylate chloride form, especially called pleximon 735 or TMAE MC 80 by Rohm, diallyldimethylammonium chloride (DADMAC), trimethyl ammonium propyl methacrylamide chloride form, especially called MAPTAC, or mixtures thereof.

The cationic modified polysaccharide may contain cationic or cationizable units resulting from a chemical transformation after polymerization of monomers precursors of cationic or cationizable functions. By way of example, mention may be made of polyp-chloromethylstyrene which, after reaction with a tertiary amine such as a trimethyl amine, forms quaternized polyparatrimethylaminomethylstyrene. Cationic or cationizable units are associated with negatively charged counterions. These counterions can be chosen from chloride ions, bromides, iodides, fluorides, sulfates, methylsulfates, phosphates, hydrogenphosphates, phosphonates, carbonates, hydrogenocarbonates, or hydroxides. Preferably, counter-ions selected from hydrogenophosphates, methylsulfates, hydroxides and chlorides are used.

The degree of substitution of the cationic modified polysaccharides used in the invention is at least 0.01 and preferably at least 0.1. If the degree of substitution is less than 0.01, the efficiency of carrying out the removal is reduced. If the degree of substitution exceeds 0.1, the polysaccharide inevitably swells in the liquid. In order to use a modified polysaccharide substituted at a level greater than 0.1, it is preferable to modify it to make it insoluble. These changes are described later.

The degree of substitution of the cationic modified polysaccharide corresponds to the average number of cationic charges per sugar unit.

Among the modifications of the polysaccharide intended to improve its affinity, mention may also be made of the introduction of hydrophilic or hydrophobic uncharged groups.

Among the hydrophilic groups that can be introduced include in particular one or more saccharide or oligosaccharide residues, one or more ethoxy groups, one or more hydroxyethyl groups or an oligoethylene oxide.

Among the hydrophobic groups which can be introduced include an alkyl group, aryl, phenyl, benzyl, acetyl, hydroxybutyl, hydroxypropyl or their mixture.

By alkyl or aryl or acetyl radical, preferentially means radicals alkyl or aryl or acetyl having 1 to 22 carbon atoms.

The degree of substitution of the modified vegetable gums with hydrophilic or hydrophobic uncharged groups used in the invention is at least 0.01 and preferably at least 0.1.

The degree of substitution of the modified polysaccharide with hydrophilic or hydrophobic non-charged groups corresponds to the average number of hydrophilic or hydrophobic non-charged groups per sugar unit.

It is possible to carry out several of the modifications proposed above for increasing the affinity of the polysaccharide for the metals to be removed on the same polysaccharide.

Among the modifications of the polysaccharide intended to make it insoluble, mention may in particular be made of the possibility of carrying out a chemical crosslinking of the polysaccharide, or of adsorbing it chemically or physically on a mineral or organic support that is insoluble in water.

Preferably, a chemical crosslinking of the polysaccharide is used to render it insoluble. The chemical crosslinking of the polysaccharide can be obtained by the action of a crosslinking agent chosen from formaldehyde, glyoxal, halohydrins such as epichlorohydrin or epibromohydrin, phosphorus oxychloride, polyphosphates, diisocyanates, bi-ethylene urea, polyacids such as adipic acid, citric acid, acrolein and the like. The chemical crosslinking of the polysaccharide can also be obtained by the action of a metal complexing agent such as, for example, zirconium (IV) or sodium tetraborate. The chemical crosslinking of the polysaccharide can also be obtained under the effect of ionizing radiation.

The level of insolubilization of the polysaccharide is satisfactory when the mass fraction of organic soluble in the polysaccharide is less than 10%. As indicated previously, the modifications intended to improve the polysaccharide affinity for the metals, and the modifications intended to make it insoluble, can be carried out separately and in the order that is desired. It may also be possible to make these changes simultaneously. By way of example, where the modifications of the polysaccharide are carried out simultaneously, mention may be made of an insoluble cationic vegetable gum obtained by placing the polysaccharide in the presence of excess epichlorohydrin and a trimethylamine. Epichlorohydrin generates in-situ a reagent carrying a quaternary ammonium which will allow to make the cationic polysaccharide on the one hand. In addition, the excess epichlorohydrin makes it possible to crosslink the polysaccharide.

The optionally modified and optionally insoluble polysaccharide of the invention may be used in the form of a powder or may be shaped into granules.

The crosslinking chemical reaction can be used to obtain insoluble granules.

The optionally modified starches may be shaped by granulation during the crosslinking reaction in order to obtain insoluble particles of the order of one millimeter (for example between 200 μm and 5 mm), which makes it possible to remove them easily from the medium to be treated.

In an industrial installation, these granulated products have the advantage of being used in column, in the same way as the exchange resins, thus offering a large exchange surface while limiting the pressure drop.

It is possible to use the optionally modified and optionally insoluble polysaccharide of the invention alone, or in a mixture with other scavengers such as, for example, exchange resins.

It is possible to mix the optionally modified polysaccharide and optionally insoluble of the invention with inert fillers such as polymer powder or sand to ballast it.

The following examples illustrate the invention without limiting its scope.

Examples:

Example of preparation of a starch according to the invention: synthesis of an insolubilized cationic starch (starch A).

In a reactor of 1 liter cylindrical double-wrapped, equipped with an anchor-type mechanical agitation, a dropping funnel and a condenser, 75 ml of demineralised water are introduced, then 750 mg of sodium chloride and 50 grams of waxy corn starch. The whole is placed under a nitrogen atmosphere and with stirring at 100 rpm. 5.2 ml of epibromohydrin are added, the mixture is stirred for 3 minutes and then 3 grams of sodium hydroxide in pellet dissolved in 20 ml of demineralized water are added. The reaction medium has a pasty, very viscous appearance. Stirring is then stopped and allowed to react at room temperature (25 ° C.) for 16 hours. At the end of this time, the reaction mass became friable. A solution of 23 grams of sodium hydroxide in pellet is added in 60 ml of demineralized water and the stirring is started again at 100 revolutions per minute. The dough breaks up and disperses in the liquid. After 30 minutes, the reaction medium is heated to 65 ^° C. Once at this temperature, 90 ml of Quab 188 (69% chlorohydroxypropyltrimethylammonium chloride in water sold by Degussa AG) are added dropwise over 30 minutes. . The addition is complete, the reactor is maintained at a temperature of 60 ^° C. with stirring for 2 hours. Stirring is then stopped and allowed to cool to room temperature. It is kept at rest for 2 hours to decant the solid. The supernatant is removed by suction using a cane with filter tip, then reintroduced 600 ml of demineralized water into the reactor. The reaction medium is brought to pH = 6 by addition of 1N hydrochloric acid. It is then placed under stirring for 2 hours. The solid + liquid mixture is then filtered on No. 3 frit. The cake is taken up in a liter of demineralised water heated at 70 ^° C. with vigorous stirring for 2 hours, at the end of which the stirring is stopped and decanted. The supernatant is removed by suction using a filter nozzle cane. The washing operation by redispersion in 1 liter of demineralized water, decantation and removal of the supernatant is repeated 4 times with cold water. At the end of the last washing, the decanting solid is separated and then frozen and dried by lyophilization.

We obtain 60 grams of very aerated white powder, which is easily absorbed in water but does not solubilize.

Elemental analysis on nitrogen shows that this product has a cationic DS of 0.12.

Examples of evaluation of a starch of the invention

In the two examples given below, the arsenic assays are performed by ICP / MS (Inductively Coupled Plasma / Mass Spectrometer) with an uncertainty of 10%. The samples to be analyzed are acidified immediately with nitric acid after collection and then stored in the refrigerator in polyethylene bottles.

Example 1

In this test, we determine the adsorption capacity of As (V) of crosslinked starch starch Amidon A at neutral pH and at a temperature of 7 ° C.

An arsenic stock solution (V) of concentration 500 mg / l is prepared from arsenic oxide As ₂ O ₅ . Girls solutions, with a concentration ranging from 1 to 50 mg of As / l, are prepared just before use by dilution of the stock solution.

For each of the daughter solutions, 42.5 mg of the starch A is injected into a 150 ml pyrex beaker at 100 ml of the solution to be treated. The pH of the suspensions is adjusted to pH 7 with concentrated NaOH and HCl solutions. After 15 hours of contact time ("equilibrium time") at 7 ° C., the supernatants of the suspensions are recovered by filtration for determination of their residual arsenic content. For filtration, we use PVDF ₁ Millex syringe filters with 0.45 μm porosity.

The results are summarized in the table below.

This test demonstrates the effectiveness of crosslinked cationic starch to remove As (V) at neutral pH and at a temperature of 7 ° C. Furthermore, it can be noted that the product has a maximum adsorption capacity of the order of 40 mg of As / gram of solid.

EXAMPLE 2 This test is carried out on natural water from the Rennes region, which has been clarified by a coagulation / flocculation treatment, and which we then doped with arsenic (V) at a level of 100 μg of As (V) / liter using an AS ₂ O ₅ arsenic oxide solution.

For this test, 42.5 mg of crosslinked cationic starch "Amidon A" to be tested in 100 ml of doped clarified water are introduced with stirring at a temperature of 70 ^° C. and after a contact time of 15 hours, the suspension is filtered using a PVDF Millex syringe filter with a porosity of 0.45 μm, for recover its supernatant and determine the residual concentrations of natural organic matter and arsenic.

The dosage of natural organic materials is carried out by u.v. spectrophotometry. at 254 nm with a Shimadzu UV-160 model 204-04550.

The results are shown in the table below.

This example shows that when it is used to treat a natural water, the crosslinked cationic starch makes it possible to eliminate a fraction of the natural organic matter but also a part of the arsenic present in this water.

Under the conditions of the test (7 ⁰ C, neutral pH, [starch] = 425 mg / l, contact time = 15h, [As (V)] ~ 100 μg / l), treatment with starch A allows to remove about 45% of the natural absorbing organic matter in UV at 254 nm and 45% of the arsenic (V).

Claims

1. Use of a composition comprising at least one polysaccharide for purifying an arsenic-laden water.

2. Use according to claim 1 characterized in that the polysaccharide is selected from the group comprising cellulose, starches and vegetable gums.

3. Use according to claim 2, characterized in that the cellulose is of plant origin, bacterial, animal, fungal or amoebic, preferably plant, bacterial or animal.

4. Use according to claim 2, characterized in that the starch is selected from wheat starch, potato starch, corn starch, sweet potato starch, tapioca starch , cassava starch, sago starch, rice starch, glutinous corn starch, waxy maize starch, high amylose corn starch or mixtures thereof.

5. Use according to claim 2, characterized in that the starch undergoes pretreatment pre-gelatinization such as cooking with hot water or steam.

6. Use according to claim 2, characterized in that the vegetable gum is selected from glucomanannes such as Konjac, xyloglucannes such as Tamarind gum, galactomannans such as guar, carob, tara, fenugreek or "mesquite" gum ", Or gum arabic or their mixtures.

7. Use according to claim 2 characterized in that the vegetable gum is chosen from galactomannans, such as guar.

8. Use according to any one of claims 1 to 7, characterized in that the polysaccharide is modified and comprises one or more cationic or cationizable groups.

9. Use according to claim 8, characterized in that the cationic or cationizable groups are selected from groups comprising quaternary ammonium or tertiary amines, pyridiniums, guanidiniums, phosphoniums or sulfoniums.

10. Use according to claim 8 or 9, characterized in that the introduction of cationic or cationizable groups in the plant derivative is carried out by a nucleophilic substitution reaction.

11. Use according to claim 8 or 9, characterized in that the introduction of cationic or cationizable groups is carried out by an esterification with amino acids such as for example glycine, lysine, arginine, acid 6- aminocaproic acid, or with quaternized amino acid derivatives such as, for example, betaine hydrochloride.

12. Use according to claim 8 or 9, characterized in that the introduction of cationic or cationizable groups is carried out by radical polymerization comprising the grafting of monomers comprising at least one cationic or cationizable group on the polysaccharide.

13. Use according to claim 12, characterized in that the monomers comprising at least one cationic or cationizable group used to carry out this radical polymerization are chosen from the compounds of formulas (I), (II), (III), ( IV) or (V) following: the compound of general formula (I)

(D in which:

AnΘ represents a CIΘ, BrΘ, IΘ, SO42Θ, CO32Θ, CH3-OSOΘΘ, OHΘ or CH3-CH₂-OSOΘΘ ion,

R1 to R5, which may be identical or different, represent, independently of one another, an alkyl group having from 1 to 20 carbon atoms, a benzyl radical or an H atom, and

n is 1 or 2, or

the compound of general formula (II)

(N) wherein:

X represents a group -NH or an oxygen atom O,

R 4 represents a hydrogen atom or an alkyl group having from 1 to 20 carbon atoms,

R 5 represents an alkene group having from 1 to 20 carbon atoms; R 1, R 2, R 3, which are identical or different, represent, independently of one another, an alkyl group having from 1 to 20 carbon atoms,

BNΘ represents a CIΘ, BrΘ, IΘ, SO42Θ, CO32Θ, CH3-OSOΘΘ, OHΘ or CH3-CH₂-OSOΘΘ ion, and n is 1 or 2, or

the compound of general formula (III)

(III)

in which :

R 1 to R 6, which are identical or different, represent, independently of one another, a hydrogen atom or an alkyl group having from 1 to 20 carbon atoms, but with one of the groups R 1 to R 6 representing a group -CH = CH 2,

CNΘ represents a CIΘ, BrΘ, IΘ, SO42Θ, CO32Θ, CH3-OSOΘΘ, OHΘ or CH3-CH₂-OSOΘΘ ion, and n is 1 or 2, or

the compound of general formula (IV)

CH ₂

CH _{3N @ /} CH ₂ _D n _Θ

N CH-CH ₂ ^S CH ₃

CH ₂ // n

(IV)

in which : DnΘ represents a CIΘ, BrΘ, IΘ, SO42Θ, CO32Θ, CH3-OSOΘΘ, OHΘ or CH3-CH₂-OSOΘΘ ion, and n is 1 or 2.

14. Use according to claim 12, characterized in that the monomers comprising at least one cationic or cationizable group used to carry out this radical polymerization are chosen from: 2-dimethylaminoethyl acrylate (ADAM), acrylate of 2 quaternized dimethylaminoethyl (ADAM-Quat), 2-dimethylaminoethyl methacrylate (MADAM), quaternized 2-dimethylaminoethyl methacrylate (MADAM-Quat), quaternized 2-diethylaminoethyl methacrylate chloride form called pleximon 735 or MAE MC 80 by the Rohm company, diallyldimethylammonium chloride (DADMAC), trimethyl ammonium propyl methacrylamide chloride form called MAPTAC, or mixtures thereof.

15. Use according to any one of claims 8 to 14, characterized in that the cationic or cationizable groups are associated with negatively charged counter-ions selected from chloride ions, bromides, iodides, fluorides, sulfates, methylsulfates, phosphates, hydrogen phosphates, phosphonates, carbonates, hydrogenocarbonates, or hydroxides.

16. Use according to any one of claims 8 to 14, characterized in that the degree of substitution of the modified vegetable gum by introduction of one or more cationic groups is at least 0.01, and preferably of at least 0.1.

17. Use according to any one of claims 1 to 16, characterized in that the polysaccharide is modified to make it insoluble.

18. Use according to claim 17, characterized in that the insoluble polysaccharide is obtained by chemical crosslinking of the vegetable gum, or by the adsorbent chemically or physically on a mineral or organic support insoluble in water.

19. Use according to claim 17, characterized in that the insoluble polysaccharide is obtained by chemical crosslinking.

20. Use according to claim 19, characterized in that the chemical crosslinking is obtained by the action of a crosslinking agent chosen from formaldehyde, glyoxal, halohydrins such as epichlorohydrin or epibromhydrin, phosphorus oxychloride. polyphosphates, diisocyanates, bisethylene urea, polyacids such as adipic acid, citric acid, acrolein and the like.

21. Use according to claim 19, characterized in that the chemical crosslinking is obtained by the action of a metal complexing agent, such as for example zirconium (IV) or sodium tetraborate.

22. Use according to claim 19, characterized in that the chemical crosslinking is obtained by the effect of ionizing radiation.

23. Use according to any one of claims 19 to 22, characterized in that the crosslinking is conducted until the mass fraction of organic soluble in the polysaccharide is less than 10%.

24. Use according to one of the preceding claims characterized in that the polysaccharide is shaped in the composition in the form of a powder or in the form of granules.

25. Use according to any one of claims 1 to 24, characterized in that the optionally modified and optionally insoluble polysaccharide of the invention is mixed with other scavengers.

26. Use according to any one of claims 1 to 25, characterized in that the optionally modified and optionally insoluble polysaccharide of the invention is mixed with inert fillers such as sand or polymer powder.